Until recently, improvements in the resolution of optical lithography processes have come largely from the use of Deep Ultra Violet (DUV) exposure sources having shorter exposure wavelengths (.lambda.). Currently available DUV exposure sources include the following two types of excimer lasers: (1) Krypton Fluoride (KrF) having a .lambda. of 248 nm and (2) Argon Fluoride (ArF) having a .lambda. of 193 nm. However, in order to fabricate device generations at 0.18 .mu.m (180 nm) and below, it has become clear to the industry that the lithography process will need to resolve feature dimensions below .lambda. if either of the two foregoing exposure sources are to be utilized.
An alternative to utilizing the foregoing excimer lasers is non-optical lithography using very short exposure wavelength sources such as Extreme Ultra Violet (EUV), X-ray, or electron beam (E-beam). Unfortunately, all the non-optical lithography technologies require some form of technological break-through combined with an adequate support infrastructure in order to become "production-worthy," or in other words, commercially feasible. While it is likely that the necessary technology break-through and infrastructure build-up will eventually occur, to date they have not. As such, for semiconductor device production having design rules in the range of 0.18 .mu.m down to 0.10 .mu.m, optical lithography is currently considered to be the most economical and preferred process technology. Accordingly, there exists a need to find innovative methods that can consistently pattern sub-.lambda. device features with optical lithography.
For sub-.lambda. device features, the mask pattern image formation is strongly dependent on the optical diffraction with the immediately adjacent patterns. For binary-type masks such as chrome patterns on a quartz glass substrate, the resolution is diffraction limited as imposed by the exposure tool. However, by introducing a .pi. phase shift as the exposure wavefront passes through the mask patterns, it has been demonstrated that the optical resolution limit can be greatly extended. Depending on the degree of phase shifting effect on the mask, it is possible to double the spatial frequency resolution for the mask patterns. In other words, the pattern resolution achievable with a phase shift mask (PSM) can reach 1/2 .lambda..
The first PSM application in optical lithography was reported by M. D. Levenson in 1982 (IEEE Trans. Electron. Devices 29, 1828, 1982). Since this time, there has been a continuous effort in the industry to explore and develop PSM technology. However, due to the inherent complexity of mask design, the learning curve for making and applying PSM has been long and arduous. Nonetheless, several forms of practical PSM technology have been developed. With regard to making line and space patterns, there are three major types: 1) alternating PSM (as originally proposed by Levenson); 2) attenuated PSM; and 3) chromeless PSM.
In accordance with alternating PSM, 0-phase and .pi.-phase alternating areas are formed between chrome mask features. There are two major unsettled issues concerning alternating PSM design. The first is the unavoidable conflict of phase assignment, and the second is the unwanted resist patterns caused by the 0 to .pi. phase transitions on the mask. The currently proposed solutions to these issues either add more complexity to the mask design or require the use of more than one mask. As such, none of the proposed solutions are attractive from a "commercialization" or "production cost" point of view.
From a design viewpoint, alternating PSM is much more challenging than attenuated PSM. Attenuated PSM typically utilizes an energy absorbing thin film layer deposited on a quartz substrate. This energy absorbing film has the property of causing a 180 degree (.pi.) phase change in the electric (E) field as the exposure wavefront passes through the mask. After mask patterns have been delineated, there is a .pi. phase shift in between the attenuated film areas and non-patterning glass only areas. Unlike the traditional chrome mask, this type of PSM typically causes some amount of attenuation by the actinic (or effective) exposure .lambda.. The extent of attenuation is mainly dependent on the phase shifting film structures and/or the interlayer thin chrome film deposited on the glass substrate. The attenuation permits a certain percent of actinic exposure .lambda. to "leak" through the phase shifting areas of the mask. Normally the amount of attenuation is described as percent transmission (%T).
FIGS. 1A-1C illustrate aerial images of a typical attenuated PSM with intensity profiles (%T) equaling 100%, 25% and 5%, respectively. As can be observed from FIGS. 1A-1C, relatively high intensity levels result from the attenuated, phase shifted areas. The strength of the intensity levels seems to be related to the %T. Specifically, the higher the %T, the stronger the intensity level. For the non-phase shifting (glass only) areas, the intensity levels remains unchanged. In order to minimize these "undesirable" intensity levels resulting from the attenuation, the standard industry practice is to limit the %T to be at most 5% for DUV exposure .lambda..
Finally, with regard to chromeless PSM, the .pi. phase shift area can be made by simply etching into the quartz substrate to a precise depth. The non-etched areas and the etched areas have an optical path difference (OPD) that can cause a .pi. phase shift as the exposure wavefront passes through the mask. Optically, the chromeless PSM concept is substantially an extension of the attenuated PSM. In other words, the chromeless PSM can be thought of as an attenuated PSM with 100% transmission. As observed in FIG. 1A, for a high %T, the "leakage" of actinic exposure .lambda. causes very strong aerial image intensity profiles.
Heretofore, the standard method for controlling the "undesirable" intensity levels is to limit the %T. Unfortunately, very low %T limits the potential resolution advantage that can otherwise be gained by using the phase shifting film. The lower the %T, the more the resulting film acts like a non-phase shifting chrome film. Accordingly, in order to achieve higher resolution, it is much more desirable to use a high %T attenuated PSM. One solution to the foregoing problem is to utilize an opaque film layer to "block" off the leaky phase shifting areas.
As shown in FIG. 2, a chrome opaque film can effectively minimize the "undesirable" intensities. The width of the chrome blocking layer needs to be smaller than the high %T attenuated phase shifting areas. To manufacture this chrome blocking layer, it is necessary to perform a second resist coating, alignment, and imaging process. This second step requires tight control of the width of the chrome blocking layer and the alignment margin in order to ensure the chrome blocking layer will be effective and not interfere with the phase shifting pattern areas.
It is clear that one disadvantage of using a chrome blocking layer is the need to perform two alignment processes for making such a reticle. The chrome blocking layer is normally imaged by an optical laser pattern generator. As such, it often suffers from lower resolution and limited alignment accuracy. In addition, this second process step adds to both the complexity and cost of the mask.
Moreover, as stated, the chrome blocking layer is utilized to "block" the "undesirable" aerial image intensities formed by the high %T phase shifting areas. As shown in FIG. 2, the remaining aerial images are mainly formed by the non-phase shifting patterns. However, the aerial image intensity levels are not as high as the ones formed by the high %T phase shifting areas. As a result, the expected resolution enhancement from the traditional chrome-blocked PSM is substantially negated.
Accordingly, there remains a need for a mask which allows for the use of the high intensity levels formed by the high %T phase shifting areas (because the higher intensity levels offer an inherent higher resolution potential), and which does not require the use of chrome blocking layers so as to reduce the overall complexity and cost of the mask.